transcriptome assembly got huge contigs

[lin]

Hi, I am a beginner of transcriptome assembly. I did a algae transcriptome assembly using spades. I don't understand why I got contigs that is over 25K bp in length. is that possible?
length. counts of contigs
100:199 149494
200:299 709026
300:399 83948
400:499 31883
500:599 16756
600:699 10221
700:799 6982
800:899 4988
900:999 3538
1000:1099 2663
1100:1199 2098
1200:1299 1627
1300:1399 1368
1400:1499 1045
1500:1599 879
1600:1699 690
1700:1799 581
1800:1899 452
1900:1999 404
2000:2099 341
2100:2199 312
2200:2299 274
2300:2399 213
2400:2499 179
2500:2599 174
2600:2699 140
2700:2799 132
2800:2899 132
2900:2999 96
3000:3099 93
3100:3199 83
3200:3299 91
3300:3399 85
3400:3499 73
3500:3599 75
3600:3699 65
3700:3799 87
3800:3899 69
3900:3999 67
4000:4099 55
4100:4199 48
4200:4299 49
4300:4399 48
4400:4499 56
4500:4599 61
4600:4699 47
4700:4799 53
4800:4899 49
4900:4999 44
5000:5099 42
5100:5199 30
5200:5299 46
5300:5399 37
5400:5499 41
5500:5599 32
5600:5699 27
5700:5799 35
5800:5899 30
5900:5999 44
6000:6099 34
6100:6199 35
6200:6299 22
6300:6399 25
6400:6499 28
6500:6599 24
6600:6699 39
6700:6799 33
6800:6899 28
6900:6999 28
7000:7099 22
7100:7199 22
7200:7299 24
7300:7399 19
7400:7499 30
7500:7599 36
7600:7699 27
7700:7799 14
7800:7899 16
7900:7999 23
8000:8099 16
8100:8199 21
8200:8299 14
8300:8399 17
8400:8499 17
8500:8599 14
8600:8699 18
8700:8799 13
8800:8899 11
8900:8999 12
9000:9099 12
9100:9199 10
9200:9299 8
9300:9399 11
9400:9499 14
9500:9599 13
9600:9699 16
9700:9799 6
9800:9899 9
9900:9999 11
10000:10099 8
10100:10199 5
10200:10299 10
10300:10399 8
10400:10499 7
10500:10599 4
10600:10699 6
10700:10799 5
10800:10899 4
10900:10999 7
11000:11099 6
11100:11199 3
11200:11299 3
11300:11399 6
11400:11499 7
11500:11599 8
11600:11699 6
11700:11799 13
11800:11899 6
11900:11999 4
12000:12099 4
12100:12199 7
12200:12299 4
12300:12399 4
12400:12499 5
12500:12599 4
12600:12699 6
12700:12799 8
12800:12899 10
12900:12999 3
13000:13099 8
13100:13199 3
13200:13299 6
13300:13399 1
13400:13499 4
13500:13599 6
13600:13699 4
13700:13799 4
13800:13899 3
13900:13999 1
14000:14099 3
14100:14199 3
14200:14299 3
14300:14399 3
14400:14499 2
14500:14599 3
14600:14699 3
14700:14799 2
14800:14899 1
14900:14999 1
15000:15099 2
15100:15199 3
15200:15299 2
15300:15399 2
15400:15499 1
15500:15599 2
15600:15699 1
15700:15799 2
15800:15899 2
15900:15999 2
16000:16099 3
16100:16199 2
16200:16299 1
16300:16399 1
16400:16499 2
16500:16599 4
16600:16699 1
16700:16799 1
16800:16899 2
17000:17099 1
17300:17399 2
17400:17499 1
17500:17599 1
17700:17799 1
17800:17899 1
17900:17999 3
18200:18299 1
18300:18399 1
18500:18599 1
18700:18799 1
18800:18899 1
18900:18999 1
19100:19199 1
19200:19299 1
19300:19399 1
19500:19599 2
19800:19899 1
19900:19999 1
20100:20199 1
20300:20399 2
20500:20599 1
20900:20999 1
21100:21199 1
21500:21599 1
21700:21799 2
21900:21999 1
22000:22099 1
23000:23099 1
23100:23199 1
24600:24699 1
25200:25299 1
25500:25599 1
26500:26599 1
26900:26999 1
27800:27899 1
28800:28899 1
31500:31599 1
38800:38899 1

Total length of sequence: 298114914 bp
Total number of sequences: 1033416
N25 stats: 25% of total sequence length is contained in the 76269 sequences >= 437 bp
N50 stats: 50% of total sequence length is contained in the 327373 sequences >= 238 bp
N75 stats: 75% of total sequence length is contained in the 657535 sequences >= 214 bp
Total GC count: 137442543 bp
GC %: 46.10 %

[pmiguel]

Unlikely that they derive from RNA. My guess would be that if you took a few kb of sequence from the largest contig and did a blastn to nt at genbank you would get a hit to chloroplast. Speculation on my part, of course...

--
Phillip